Loans and Allowance for Loan Losses - Schedule of Risk Category of Commercial and Commercial Real Estate Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Risk Category Of Loans [Line Items]
Total	$ 2,949,277	$ 2,717,269	$ 2,049,054
Commercial [Member]
Risk Category Of Loans [Line Items]
Total	2,949,277	2,717,269
Commercial Real Estate [Member] | Construction [Member]
Risk Category Of Loans [Line Items]
Total	542,228	374,306
Commercial Real Estate [Member] | Other [Member]
Risk Category Of Loans [Line Items]
Total	3,939,326	3,980,246
Pass [Member] | Commercial [Member]
Risk Category Of Loans [Line Items]
Total	2,789,061	2,577,824
Pass [Member] | Commercial Real Estate [Member] | Construction [Member]
Risk Category Of Loans [Line Items]
Total	505,930	357,438
Pass [Member] | Commercial Real Estate [Member] | Other [Member]
Risk Category Of Loans [Line Items]
Total	3,734,727	3,762,896
Criticized [Member] | Commercial [Member]
Risk Category Of Loans [Line Items]
Total	79,866	74,876
Criticized [Member] | Commercial Real Estate [Member] | Construction [Member]
Risk Category Of Loans [Line Items]
Total	22,130	14,758
Criticized [Member] | Commercial Real Estate [Member] | Other [Member]
Risk Category Of Loans [Line Items]
Total	79,170	98,451
Classified - Substandard [Member] | Commercial [Member]
Risk Category Of Loans [Line Items]
Total	43,835	37,367
Classified - Substandard [Member] | Commercial Real Estate [Member] | Other [Member]
Risk Category Of Loans [Line Items]
Total	58,364	58,584
Classified - Nonaccrual [Member] | Commercial [Member]
Risk Category Of Loans [Line Items]
Total	28,151	24,798
Classified - Nonaccrual [Member] | Commercial Real Estate [Member] | Construction [Member]
Risk Category Of Loans [Line Items]
Total	14,168	2,110
Classified - Nonaccrual [Member] | Commercial Real Estate [Member] | Other [Member]
Risk Category Of Loans [Line Items]
Total	30,624	30,108
Classified - Doubtful [Member] | Commercial [Member]
Risk Category Of Loans [Line Items]
Total	8,364	2,404
Classified - Doubtful [Member] | Commercial Real Estate [Member] | Other [Member]
Risk Category Of Loans [Line Items]
Total	$ 36,441	$ 30,207